Thousands of people are fighting to solve a mystery. How did a large passenger plane, 239 people onboard, simply vanish? At this hour, investigators working in a race against time to find any clue. Why was there no S.O.S.? One minute, a plane tracked by radar on this route. And then, at this point, right there, it was simply gone. And tonight, it's a daunting task for the search teams, finding any scrap of debris on two sides of land, an area roughly the size of California and growing by the hour. ABC's bob woodruff is on the ground in beijing, tonight, on the dramatic disappearance. Reporter: The plane has simply vanished. Today, in the waters of the south China sea, the search zone was expanded. But no clues. 34 helicopters and planes, along with 40 ships, including submarines. Even sites that monitor nuclear weapons have been asked if they saw an explosion the night the plane disappeared. While on land, agony and demands for answers from the families of the 227 passengers and 12 crew, including 3 Americans, a father and businessman from Texas, and 2 small children. Flying from Malaysia to beijing, China, it all began one hour in, during the safest part of the flight. The night sky was clear. Plane's safety record excellent. Veteran pilot. When, the plane disappears. Officials, today, are saying an oil slick that they tested was not from this flight. Nor was a yellow object floating in the sea. How could this be? The flight was tracked by radar, as it soared out into the ocean. But that commercial radar only reaches about 250 miles into the sea. Malaysian military radar may have picked up the plane, saying it might have turned back. But tonight, that is not clear. And why hasn't the plane been found? The search zone is massive. So, if the plane left its route, just turning a little to the left or right, the search zone expands by hundreds, if not a thousand miles. The plane does have black boxes that ping out a noise. And the waters are shallow, 165 feet deep. But submarines and special ships need to be within five to ten miles to hear it. Today, investigators searched for clues on the two passengers who boarded using stolen passports. Those two passengers bought their tickets at the same time, at this travel agency. Suspicious but few answers. The key tonight, is why was there not a distress call? The pilots would have had the ability to call in by radio. But they did not. But did something so catastrophic happen, that they simply had no time? All of that is still a mystery. Diane? Thank you, bob, reporting in from beijing for us tonight. As bob said, we turn to two big possibilities. Was there terrorism or a problem with the plane? Structural or pilot error? We have two reports, next, starting with Pierre Thomas, questioning his sources about terrorism. Pierre? Reporter: Tonight, the FBI is urgently trying to find out why those imposters, one posing as this Italian man, used stolen passports to get an Malaysia airlines flight 370. The FBI is trying to compare fingerprints and those at the airport to those of known terrorists. The fingerprints and stolen passports represent tantalizing clues. But the passports does not indicate terrorism. They are often used by drug smugglers and other criminals. There's at least 40,000 stolen passports every year. It leaves us very vulnerable, not just to terrorist attacks. But it facilitates other criminal activity. Reporter: Police in Thailand are questioning the owners of these travel agencies, which sold the one-way tickets used by the imposters. ABC news learned they were purchased at the same time by an iranian man named Mr. Ali. Some believe that suggests the men coordinated their activities. Another clue, the surveillance video of the suspects moving through the airport. FBI officials hope to get that, too, so they can study the luggage of the imposters and their demeanor. Tracking it tonight. Thank you, Pierre. And the other possibility, a malfunction in the plane. So sudden, so catastrophic, there's no chance to call for help. Was weather involved? Or human error? ABC's David Kerley talked to the experts. Reporter: Large passenger jetliners do not just disappear. Nothing causes an airplane to come down from 35,000 feet, other than things that can be catastrophic. Reporter: And the list is long. A total structural failure. Loss of both engines. Complete electrical failure. Or pilot error. In fact, a ten-year study by Boeing showed that nearly two-thirds of aircraft losses are due to human factors. Even without wreckage, investigators should be interviewing everyone who touched that plane. Those who loaded food. Pilots who flew this jet on earlier flights, maintenance records, cargo handlers. In fact , it was improperly stored cargo that led to a fire that brought down a valujet in the Florida everglades. So, we need to keep an open mind, continue the search, find those black boxes and begin to solve this mystery. Reporter: Those two black boxes, which record the pilot's voices and data from the plane can be critical. After nearly two years, those from air France 447 were recovered from the bottom of the atlantic by a mini sub. They showed that a sensor malfunction followed by pilot error led to the crash. Recorders tell us what happened, but sometimes not why. That was the case with twa 800, which went down off long Island. It was an explosion. But it wasn't until all the recovered debris was reassembled that investigators could tell by bent supports that the center fuel tank exploded. Ignited by a frayed wire. But tonight, with so few facts, flight 370 remains in the category of unsolved mysteries. It's unlikely it will remain a mystery. Every crashed passenger plane in recent history has been found. And the pieces of debris, a piece of skin or a seat can yield a lot of clues. But time pressure is on, Diane. The batteries for the all-important black boxes only last about 30 days.

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